Convective hot air impingement device with localized return paths

ABSTRACT

This is a pre-heater system especially useful in an electrostatic marking apparatus. It is used to heat the toner and paper (media) prior to the paper entering the conventional fuser station. Using this pre-heater increases fusing productivity (ppm), widens media latitude and/or extends fuser member life for a given fuser system. It also dramatically improves uniform heat transfer and paper handling problems encountered in prior art preheat fusing assemblies. The preheater is used together with the conventional fuser stations used today in electrostatic marking systems.

This invention relates to electrostatic printing systems and, morespecifically, to the fusing assembly and a pre-heater for said assembly.

BACKGROUND

Generally, in a commercial electrostatographic reproduction apparatus(such as copier/duplicators, printers, or the like), a latent imagecharge pattern is formed on a uniformly charged photoconductive ordielectric member. Pigmented marking particles (toner) are attracted tothe latent image charge pattern to develop such image on the dielectricmember. A receiver member, such as paper, is then brought into contactwith the dielectric member and an electric field applied to transfer themarking particle developed image to the receiver member from thedielectric member. After transfer, the receiver member bearing thetransferred image is transported away from the dielectric member and theimage is fixed or fused to the receiver member by heat and/or pressureto form a permanent reproduction thereon.

Sometimes copies made in Xerographic or electrostatic imaging systemshave defects caused by improper fusing of the marking material or tonerto the receiving media such as paper. There can be many possible causesof these defects including toner contaminants, charging problems orincomplete fusing. In these systems, the image is fixed to the receivingmember by heat and pressure to form a permanent reproduction thereon.

Typical electrographic reproduction apparatus includes at least oneheated roller having an aluminum core and an elastomeric cover layer,and at least one pressure roller in nip relation with the heated roller.The fusing device rollers are rotated to transport a receiver memberbearing a marking particle image, through the nip between the rollers.The toner or pigmented marking particles of the transferred image on thesurface of the receiver member soften and become tacky in the heat.Under the pressure, the softened tacking marking particles attach toeach other and are partially imbibed into the interstices of the fibersat the surface of the receiver member. Accordingly, upon cooling, themarking particle image is permanently fixed to the receiver member.

In some instances, low melting point marking particles or toner aresubject to increase image offset to the heating roller. This can producestreaks and undesirable defects in the final copy. This image offset canbe reduced by application of fusing oil to the heating roller. The useof such oil introduces further complications into the fusing system,such as handling of the oil and making sure that the layer of oil on theroller is uniform. Alternatively, a mechanical arrangement for reducingimage offset, without the need for fusing oil has been found. Suchmechanical arrangement provides an elongated web which is heated to meltthe marking particles and then cooled to cool the particles andfacilitate ready separation of the receiver member with the markingparticle image fixed thereto from the elongated web. The nature ofoperation of the elongated web arrangement also serves to increase theglossiness of the fixed marking particle image. As a result, sucharrangement is particularly useful for multi-color image fusing, but isnot particularly suitable for black image fusing. There is a need for aconvenient and effective way to improve the fusing operations inXerographic systems.

An option that does not involve core fuser redesign is the preheatconcept. By heating the media or paper and toner prior to entering theconventional fusing nip, it is possible to increase productivity for agiven fusing system, or if desired, trading speed benefits for lowertemperature and/or wider media latitude at current throughputs. Afterconsiderable study, it was decided to pursue hot air impingement as thepreferable means of preheating . Radiant heating was eliminated due tothe risk of fire. Conduction was not used since touching unfused printstypically lead to image defects. Convective heating is much safer thanradiant and much more “image-friendly” than conduction. This inventionintroduces a design concept that directly addresses the fundamentalissues of heat transfer efficiency, uniformity and paper handling.

SUMMARY

Higher productivity, increased reliability and lower run costs are goalsinherent to most electrostatic marking programs. Fusing systems manytimes hit technology “break-points” limiting their extensibility for usein follow-on products. The ability to heat the toner and mediaimmediately prior to entering the fuser has proven to extend a fuser'susability as a function of speed, media latitude and/or life.

This invention outlines a convective (forced hot air impingement) deviceused to raise the incoming sheet temperature and effectively share thefusing function with the conventional fuser.

A unique aspect of embodiments of this concept is the use of localizedreturn paths (straws or tubes) which dramatically improve heat transferand paper handling problems that have inhibited the functionality ofprevious “perimeter/skirt-return” designs.

This invention outlines the benefits and potential embodiments of a hotair impingement device with localized return paths to augmenteffective/uniform heat transfer and also promote stable media transport.Two primary issues with prior art such as the U.S. Pat. No. 6,754,457system current convective preheat hardware are:

1) non-uniform h (convective heat transfer coefficients) across theimpingement zone; This is at least partially caused by the reduction indirect impingement as a result of bulk air flow obstructing the directflow from jets other than near the center of the plenum (See FIG. 4 ofdrawings). Non-uniform impingement equates to non-uniform heat transfer.FIG. 4 of the drawings demonstrates how the bulk air flow (impinged airheading toward the skirt) of the skirt-return design negatively affectsthe h uniformity. While the drawings illustrate the consistent h patternthat can be expected, regardless of position inboard/outboard or crossprocess, when using the proposed localized return path geometry. FIGS. 3and 4 are side views comparing the simplified air flow patterns of thetwo designs, i.e. present and prior art. 2) The “sucking-up” of paperinto the perimeter return paths (element 20 in FIG. 4) resulting inpaper jams or image disturbance. In order to increase h for a givengeometry it is required to increase the air flow. However, increasingair flow in a skirt-return system augments the dysfunction of papersucking-up.

Proposed is a plenum design that both maintains hold specificationcritical to effective impingement (pattern, diam, etc.) and incorporateslow area vacuum return paths. The plenum geometry (although notoptimized) has been designed using empirically supported numericalmodeling. The FIG. 3 description lists plenum dimensions that wereidentified in one embodiment to nominally meet known preheat criteriasuch as h at the sheet and vacuum pressure.

The models tested have predicted average h values comparable to thosemaximum values generated using existing “open-plenum/skirt return”hardware. The air return path (vacuum area presently represents −20 mmof the process direction length under the plenum (10 mm skirt atentrance and exit.) This ID proposes that the vacuum spread across theentire plenum area via small, evenly spaced holes. Uniformly spreadingout of the return flow greatly reduces the risk of the sheet lifting offof the transport. The design reduces the maximum pulling force on thesheet by more than 10×.

The returned air is drawn through the impingement chamber via an arrayof tubes or straws. The straws open to an upper chamber where the bulkair is then fed back into the blowers, directed through heaters andfinally to the impingement plenum. This ideally closed system minimizesenergy losses. Hardware has been designed and fabricated for empiricaltesting.

In U.S. Pat. No. 6,754,457 (Ciaschi) a pre-heater assembly is disclosed.In the present invention the use of impingement holes or apertures haveproven through numerical and empirical modeling to be a much moreefficient way to convectively transfer heat (for the same flow rate).This fact has been well documented in numerous occasions. With aperturesor holes the impingement flow is more localized which is why much higherh's (convective heat transfer coefficients) are observed. This is afundamental performance advantage over the prior art including Ciaschi'ssystem. The combination of holes or apertures and localized tube airreturn paths are a more efficient transfer of heat to the media thenslots with localized return paths of the prior art. Considering theentire area within the heating zone, the apertures or holes of thepresent invention will transfer heat more uniformly. In the presentinvention, hundreds of holes may be closely spaced across the entirearea where as only a small number of slits are described in prior artsystems.

By “conventional fuser station” is meant throughout this disclosure andclaims, any of the fuser stations used in Xerox and other electrostaticcopiers or duplicators or printers used today.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an electrophotographic fusing station with apre-heater unit positioned before the fuser assembly.

FIG. 2 is a side perspective view of an embodiment of the pre-heatersystem of this invention.

FIG. 3 is a side view of an embodiment of the pre-heater system of thisinvention showing the air circulation during use.

FIG. 4 is a side view of a prior art pre-heater unit showing the aircirculation during use.

DETAILED DISCUSSION OF DRAWINGS AND PREFERRED EMBODIMENTS

In FIG. 1, the pre-heater unit 1 is shown at a position in the travelpath in an electrophotographic marking system where it is always locatedbefore the fuser roll 2 and pressure roll 3. The pre-heater 1 providesfor impingement of heater air onto a receiving member 4 having unfusedmarking particles thereon. The pre-heater 1 enhances the quality andspeed of fusing a toner or marking material to receiving member 4. Thefusing assembly made up of a fuser roll 2 and a pressure roll 3 providesthe final fusing of the toner to the paper 4. Generally, a conventionalfuser roll 2 is used usually having an elastomeric outer layer on a heatconductive roll usually made from aluminum. A heater is provided for thefuser roll either internally or externally. The receiving member, afterpassing under the pre-heater 1, is transported by a belt 5 (or a drum)to a fusing nip 6 between the fuser roll 2 and pressure roll 3 foroptimum fusing efficiency. The pre-heater 1 of this invention providesfor pre-heating paper 4 by passing a continuous uniform flow of hot airon the image-bearing surface of paper 4. Details of pre-heater 1 are setout in the description of FIG. 2. Appropriate sensors may be used alongthe paper path to provide information and conditions for high efficiencyfusing. A blower fan 7 to force heated air through the pre-heater unit 1is provided.

In FIG. 2, an embodiment of the present pre-heater 1 invention isillustrated. The pre-heater 1 comprises a substantially airtight housing8 having in an operational arrangement, an upper chamber 9 and a lowerchamber 10. These two chambers 9 and 10 are co-extensive. A plurality ofhot air impingement apertures or holes 11 are located in a bottomsection 12 of said lower chamber 10. A plurality of open ended returnvacuum tubes 13 extend from the upper chamber 9 through the lowerchamber 10 and even with or beyond the lower face of bottom section 12of the lower chamber 10. A heater coil 14 is adapted to heat the air andhave it circulated by a fan or blower 7. The air is blown into the lowerchamber 10 and through the impinging apertures 11 to uniformly heat thereceiving member or paper 4 as it travels under the pre-heating unit 1toward a “conventional” fusing station comprising a fuser roll 2 and apressure roll 3. The return vacuum tubes 13 are enabled to continuouslyrecycle the heated air to the upper chamber 9 and to the fan blower 7.The use of impingement holes or apertures 11 have proven to be a muchmore efficient way to convectively transfer heat (for the same flowrate). This is because with a plurality of holes 11 the impingement flowis more localized which is why much higher h's (convective heat transfercoefficients) are observed. This is one very fundamental advantage overthe prior art pre-heaters. The combination of apertures 11 and localizedreturn paths via tubes 13 are considered to much more efficientlytransfer heat to the receiving media or paper 4 than prior art slotswith localized return paths. Considering the entire area within theheating zone, the apertures 11 transfer heat more uniformly. In thepresent invention, hundreds of apertures 11 are closely spaced acrossthe entire area whereas only a small number of slits are used in theprior art.

The return tubes 13 in an embodiment have a diameter of from 2-10 timesthe diameter of apertures 11. Of course, these diameters will varydepending upon the size of the pre-heater 1 and other variables. In oneembodiment tested, a vacuum tube 13 diameter of 3 mm was used withapertures 11 having a diameter of about 1 mm. Openings 15 are providedin a lower section of upper chamber 9 and facilitates the return passageof heated air from the paper 4 surface to the blower fan 7 forrecirculation to the lower chamber 10 and through apertures 11. Thesereturn tubes 13 provide the sole air return to the upper chamber 9 andblower fan 7. The arrows 16 indicate the flow of heated air during useof the pre-heater 1. A hot air conduit 17 provides for passage ofpressurized heated air from the blower fan 7 to the lower chamber 10.Thus, there is hot air impingement onto paper 4 from lower chamber 10(via apertures 11) which is pressurized by blower fan 7 to the paper.Then the return heated air flows into upper chamber 9 and back to blowinlet 18. The process sequence is: (1) air is blown across heater coil14 and into lower chamber 10; (2) pressurized air is forced out holes11; (3) air heats paper 4; (4) air is returned through vacuum cylinders13; and (5) return air is collected in upper chamber 9 and fed back intoblower inlet 7.

In FIG. 3, a side view of pre-heater 1 shows the heated air circulationthroughout substantially airtight housing 8. The air passes from lowerchamber 10 through apertures 11 to the image surface of paper 4. Oncethe heated air contacts paper 4, it is recirculated through vacuum tubes13 via openings 15 to upper chamber 9 and to the blower inlet 18 (seeFIG. 2). The arrows 16 show the paths of the heated air in thepre-heater 1. Note that there is much more uniformity of heat contact onpaper 4 than the prior art systems of FIG. 4. An embodiment with typicalillustrative (not limiting) parameters and measurements would be asfollows: p=plenum height=25.4 mm; T=plate thickness=1.5 mm; D1=holediameter in=1 mm; D2=hole diameter out=3 mm and G=gap (to paper)=10 mm.

In FIG. 4, a prior art air flow having non-uniform air impingement isshown. Note that arrows 19 clearly show the non-uniformity of heated aircontact with paper 4. Also, end perimeter returns 20 of the prior arthave caused paper 4 end or edge bending because of the high pressure atreturns 20. This paper edge deforming frequently causes paper jams andother paper transport problems.

While particular embodiments have been described, alternatives,modifications, variations, improvements, and substantial equivalentsthat are, or, may be presently unforeseen, may arise to applicants orothers skilled in the art. Accordingly, the appended claims as filed andas they may be amended are intended to embrace all such alternatives,modifications, variations, improvements and substantial equivalents.

1. A pre-heater system for a fusing assembly used in an electrostaticmarking apparatus comprising in an operative arrangement a heaterhousing, an upper chamber, a lower chamber, open-ended return vacuum airtubes, hot air impingement air apertures, and at least one heater with ablower fan, said fan enabled to blow air across said heater and intosaid lower chamber, whereby pressurized heated air is thereby forced outof said apertures onto paper (or receiving media) that is adjacent saidimpingement apertures, and said vacuum air tubes extending from belowsaid heater housing to said upper chamber and enabled to return saidheated air to said upper chamber where it is fed back into said blowerfan.
 2. The system of claim 1 wherein said heated air is continuouslyfed through said impingement apertures, contacts said paper, and isreturned to said blower fan via said return vacuum tubes.
 3. The systemof claim 1 wherein said pre-heater system is located in an electrostaticmarking apparatus at a position before said fusing assembly.
 4. Thesystem of claim 1 wherein said return vacuum air tubes have a diameterat least two times the diameter of said apertures.
 5. The system ofclaim 1 wherein said pre-heater system is enabled to increase a fusingspeed of an image by at least 50%.
 6. The system of claim 1 wherein saidpre-heater system is enabled to share a fusing function with aconventional fuser station.
 7. The system of claim 1 wherein said heateris a heat coil over which said air is blown by said fan.
 8. The systemof claim 1 wherein said pre-heater system is adapted to continuouslyblow heated air uniformly across a paper surface before said paperenters a conventional fusing station.
 9. The system of claim 1 whereinsaid apertures and return tubes are arranged in a uniform pattern acrossan entire bottom section of said lower chamber.
 10. A pre-heater systemadapted to be used on a electrostatic marking apparatus at a location insaid apparatus prior to a conventional fuser roll assembly or station,said system comprising in an operative arrangement a heater housing, anupper chamber, a lower chamber, open-ended return vacuum air tubes, hotair impingement apertures and at least one heater coil with an adjacentblower fan, said upper chamber and said lower chamber located withinsaid heater housing in a contiguous arrangement, said fan adapted toblow pressurized hot air into said lower chamber, said hot air enabledto pass from said lower chamber into and through impingement aperturesto a surface of an image receiving media in order to substantiallydispense hot air throughout substantially an entire paper or mediasurface, said open-ended return vacuum air tubes enabled to subsequentlysuck up said heated air and return it to said upper chamber where it isfed to said blower fan for continuous circution to and from said mediasurface.
 11. The system of claim 10 wherein said pre-heater system islocated in an electrostatic marking apparatus at a position before saidconventional fuser roll assembly or station.
 12. The system of claim 10wherein said return vacuum air tubes have a diameter at least two timesthe diameter of said apertures.
 13. The system of claim 10 wherein saidpre-heater system is enabled to increase a fusing speed of an image on amedia by at least 50%.
 14. The system of claim 10 wherein saidpre-heater system is enabled to share a fusing function with aconventional fuser assembly or station.
 15. The system of claim 10wherein said pre-heater system is adapted to continuously blow heatedair uniformly across a paper surface before said paper enters aconventional fusing station.
 16. The system of claim 10 wherein saidapertures and return tubes are arranged in a uniform pattern across anentire bottom section of said lower chamber.
 17. A pre-heater system fora fusing assembly of an electrostatic marking apparatus comprising in anoperative arrangement an air-tight heater housing, an upper chamber insaid housing, a lower chamber coextensively below said upper chamber insaid housing, a plurality of hot air impingement apertures located in abottom section of said lower chamber, a plurality of open-ended returnvacuum tubes extending from said upper chamber, through said lowerchamber and even with or beyond said bottom section of said lowerchamber, at least one heater coil having a blower fan adapted to blowheated air into said lower chamber and through said impingementapertures to thereby uniformly contact a surface of a paper receivingmedia as it travels along a travel path to a conventional fusingstation, said return vacuum tubes enabled to continuously recycle saidheated air to said upper chamber and said fan, said pre-heater systemadapted to facilitate more efficient fusing of toner to said media, saidreturn vacuum tubes having a diameter of from 2-10 times the diameter ofsaid apertures, said heated air pressurized by said blower fan andenabled to uniformly contact via said apertures substantially an entireadjacent image-receiving surface and said media, an air opening isprovided in a lower section of said upper chamber and said blower fan,said open-ended return vacuum tubes providing the sole air return tosaid upper chamber and blower fan.